1. Field of the Invention
The present invention relates to a weld detector to detect a weld of metal material by using electromagnetic ultrasonic waves.
2. Description of the Prior Art
In recent years, with the advancement of welding technique the reliability of the seam of welded steel pipes has greately improved. Since the welded steel pipes are inexpensive compared to the seamless steel pipes, they have come to be widely used as oil well pipes or line pipes. In the fabricating process of the welded steel pipes an inspection is made on the weld of the pipe. The test on the weld includes a water pressure test and an ultrasonic test and in the ultrasonic test the weld must be set at a specified position. The positioning of the weld is done by placing the steel pipe on two turning rollers to rotate the pipe at low speed. To make the weld come to a specified position it is necessary to detect the weld.
Conventionally the detection of the weld has been performed by visual inspection. It is, however, not an easy task even with an experienced inspector to find the weld by checking the steel pipe surface usually and therefore the work efficiency is inevitably low. Thus the automatic detection of the weld has strongly been desired.
In addition to the seamed pipes, the detection of the weld is necessary in adjusting the speed of rollers of the continuous rolling mill in which the steel sheets are welded and rolled continuously.
A known method of automatically detecting the weld is to propagate ultrasonic waves in the metal material and measure the attenuation of the reflected ultrasonic wave to locate the welded portion. This method is described in the specification of Japanese Patent Application (Laid-open No. 50-36186 (1975)), titled "Method and Device for Detecting a Weld of Steel Material". This specification presents a method of distinguishing the weld from the mother plate by putting a piezo-electric type ultrasonic detector in contact with the metal material being checked with a contact medium (water) therebetween, propagating ultrasonic waves into the metal material and measuring the attenuation of the reflected ultrasonic wave.
However, the result of our experiment using the above method showed that almost no difference was observed in the attenuation between the welded and the non-welded portion and therefore the welded portion could not be located.